Add terminal trick

Record a terminal session

README.md

@@ -1,7 +1,7 @@
 # Bash-Oneliner
 I am glad that you are here! I was working on bioinformatics a few years ago and was amazed by those single-word bash commands which are much faster than my dull scripts, time saved through learning command-line shortcuts and scripting. Recent years I am working on cloud computing and I keep recording those useful commands here. Not all of them is oneliner, but i put effort on making them brief and swift. I am mainly using Ubuntu, Amazon Linux, RedHat, Linux Mint, Mac and CentOS, sorry if the commands don't work on your system.
 
-This blog will focus on simple bash commands for parsing data and Linux system maintenance that i acquired from work and LPIC exam. I apologize that there are no detailed citation for all the commands, but they are probably from dear Google and Stackoverflow.
+This blog will focus on simple bash commands for parsing data and Linux system maintenance that i acquired from work and LPIC exam. I apologize that there are no detailed citation for all the commands, but they are probably from dear Google and Stack Overflow.
 
 English and bash are not my first language, please correct me anytime, thank you.
 If you know other cool commands, please teach me!
@@ -12,13 +12,13 @@ Here's a more stylish version of [Bash-Oneliner](https://onceupon.github.io/Bash
 
 - [Terminal Tricks](#terminal-tricks)
 - [Variable](#variable)
+- [Math](#math)
 - [Grep](#grep)
 - [Sed](#sed)
 - [Awk](#awk)
 - [Xargs](#xargs)
 - [Find](#find)
 - [Condition and Loop](#condition-and-loop)
-- [Math](#math)
 - [Time](#time)
 - [Download](#download)
 - [Random](#random)
@@ -33,22 +33,23 @@ Here's a more stylish version of [Bash-Oneliner](https://onceupon.github.io/Bash
 
 #####  Using Ctrl keys
 ```
+Ctrl + a : move to the beginning of line.
+Ctrl + d : if you've type something, Ctrl + d deletes the character under the cursor, else, it escapes the current shell.
+Ctrl + e : move to the end of line.
+Ctrl + k : delete all text from the cursor to the end of line.
+Ctrl + l : equivalent to clear.
 Ctrl + n : same as Down arrow.
 Ctrl + p : same as Up arrow.
+Ctrl + q : to resume output to terminal after Ctrl + s.
 Ctrl + r : begins a backward search through command history.(keep pressing Ctrl + r to move backward)
 Ctrl + s : to stop output to terminal.
-Ctrl + q : to resume output to terminal after Ctrl + s.
-Ctrl + a : move to the beginning of line.
-Ctrl + e : move to the end of line.
-Ctrl + d : if you've type something, Ctrl + d deletes the character under the cursor, else, it escapes the current shell.
-Ctrl + k : delete all text from the cursor to the end of line.
-Ctrl + x + backspace : delete all text from the beginning of line to the cursor.
 Ctrl + t : transpose the character before the cursor with the one under the cursor, press Esc + t to transposes the two words before the cursor.
-Ctrl + w : cut the word before the cursor; then Ctrl + y paste it
 Ctrl + u : cut the line before the cursor; then Ctrl + y paste it
-Ctrl + _ : undo typing.
-Ctrl + l : equivalent to clear.
+Ctrl + w : cut the word before the cursor; then Ctrl + y paste it
+Ctrl + x + backspace : delete all text from the beginning of line to the cursor.
 Ctrl + x + Ctrl + e : launch editor defined by $EDITOR to input your command. Useful for multi-line commands.
+Ctrl + z : stop current running process and keep it in background. You can use `fg` to continue the process in the foreground, or `bg` to continue the process in the background.
+Ctrl + _ : undo typing.
 ```
 ##### Change case
 ```bash
@@ -57,8 +58,9 @@ Esc + u
 Esc + l
 # converts text from cursor to the end of the word to lowercase.
 Esc + c
-# converts letter under the cursor to uppercase.
+# converts letter under the cursor to uppercase, rest of the word to lowercase.
 ```
+
 ##### Run history number (e.g. 53)
 ```bash
 !53
@@ -69,7 +71,6 @@ Esc + c
 !!
 # run the previous command using sudo
 sudo !!
-# of course you need to enter your password
 ```
 
 ##### Run last command and change some parameter using caret substitution (e.g. last command: echo 'aaa' -> rerun as: echo 'bbb')
@@ -98,16 +99,16 @@ sudo !!
 ##### Bash globbing
 ```bash
 # '*' serves as a "wild card" for filename expansion.
-/b?n/?at      #/bin/cat
-
-# '?' serves as a single-character "wild card" for filename expansion.
 /etc/pa*wd    #/etc/passwd
 
-# ‘[]’ serves to match the character from a range.
+# '?' serves as a single-character "wild card" for filename expansion.
+/b?n/?at      #/bin/cat
+
+# '[]' serves to match the character from a range.
 ls -l [a-z]*   #list all files with alphabet in its filename.
 
-# ‘{}’ can be used to match filenames with more than one patterns
-ls {*.sh,*.py}   #list all .sh and .py files
+# '{}' can be used to match filenames with more than one patterns
+ls *.{sh,py}   #list all .sh and .py files
 ```
 
 ##### Some handy environment variables
@@ -119,6 +120,7 @@ $?   :most recent foreground pipeline exit status.
 $-   :current options set for the shell.
 $$   :pid of the current shell (not subshell).
 $!   :is the PID of the most recent background command.
+$_   :last argument of the previously executed command, or the path of the bash script.
 
 $DESKTOP_SESSION     current display manager
 $EDITOR   preferred text editor.
@@ -130,14 +132,49 @@ $USER   current username
 $HOSTNAME   current hostname
 ```
 
+##### Using vi-mode in your shell
+```bash
+set -o vi
+# change bash shell to vi mode
+# then hit the Esc key to change to vi edit mode (when `set -o vi` is set)
+k
+# in vi edit mode - previous command
+j
+# in vi edit mode - next command
+0
+# in vi edit mode - beginning of the command
+R
+# in vi edit mode - replace current characters of command
+2w
+# in vi edit mode - next to 2nd word
+b
+# in vi edit mode - previous word
+i
+# in vi edit mode - go to insert mode
+v
+# in vi edit mode - edit current command in vi
+man 3 readline
+# man page for complete readline mapping
+```
+
 ## Variable
 [[back to top](#handy-bash-one-liners)]
 ##### Variable substitution within quotes
 ```bash
 # foo=bar
+echo $foo
+# bar
+echo "$foo"
+# bar
+# single quotes cause variables to not be expanded
+echo '$foo'
+# $foo
+# single quotes within double quotes will not cancel expansion and will be part of the output
 echo "'$foo'"
 # 'bar'
-# double/single quotes around single quotes make the inner single quotes expand variables
+# doubled single quotes act as if there are no quotes at all
+echo ''$foo''
+# bar
 ```
 ##### Get the length of variable
 ```bash
@@ -178,9 +215,11 @@ echo ${var[@]#0}
 ```bash
 {var//a/,}
 ```
+
+##### Grep lines with strings from a file (e.g. lines with 'stringA or 'stringB' or 'stringC')
 ```bash
 # with grep
- test="god the father"
+test="stringA stringB stringC"
 grep ${test// /\\\|} file.txt
 # turning the space into 'or' (\|) in grep
 ```
@@ -199,15 +238,21 @@ eval "$cmd"
 echo "$bar" # foo
 ```
 
+##### Record a terminal session
+```bash
+# https://github.com/asciinema/asciinema
+asciinema rec demo.cast
+```
+
 ## Math
 [[back to top](#handy-bash-one-liners)]
 ##### Arithmetic Expansion in Bash (Operators: +, -, *, /, %, etc)
 ```bash
 echo $(( 10 + 5 ))  #15
 x=1
-echo $(( x++ )) #1 , notice that it is still 1, since it's post-incremen
+echo $(( x++ )) #1 , notice that it is still 1, since it's post-increment
 echo $(( x++ )) #2
-echo $(( ++x )) #4 , notice that it is not 3 since it's pre-incremen
+echo $(( ++x )) #4 , notice that it is not 3 since it's pre-increment
 echo $(( x-- )) #4
 echo $(( x-- )) #3
 echo $(( --x )) #1
@@ -265,10 +310,10 @@ echo "var=5;--var"| bc
 #####  Type of grep
 ```bash
 grep = grep -G # Basic Regular Expression (BRE)
-fgrep = grep -F # fixed text, ignoring meta-charachetrs
+fgrep = grep -F # fixed text, ignoring meta-characters
 egrep = grep -E # Extended Regular Expression (ERE)
-pgrep = grep -P # Perl Compatible Regular Expressions (PCRE)
 rgrep = grep -r # recursive
+grep -P # Perl Compatible Regular Expressions (PCRE)
 ```
 
 #####  Grep and count number of empty lines
@@ -280,15 +325,15 @@ grep -c "^$"
 ```bash
 grep -o '[0-9]*'
 # or
-grep -oP '\d'
+grep -oP '\d*'
 ```
 #####  Grep integer with certain number of digits (e.g. 3)
 ```bash
-grep ‘[0-9]\{3\}’
+grep '[0-9]\{3\}'
 # or
-grep -E ‘[0-9]{3}’
+grep -E '[0-9]{3}'
 # or
-grep -P ‘\d{3}’
+grep -P '\d{3}'
 ```
 
 #####  Grep only IP address
@@ -395,14 +440,14 @@ grep 'A\|B\|C\|D'
 grep 'A.*B'
 ```
 
-##### Regex any singer character (e.g. ACB or AEB)
+##### Regex any single character (e.g. ACB or AEB)
 ```bash
 grep 'A.B'
 ```
 
 ##### Regex with or without a certain character (e.g. color or colour)
 ```bash
-grep ‘colou?r’
+grep 'colou\?r'
 ```
 
 ##### Grep all content of a fileA from fileB
@@ -456,9 +501,10 @@ sed 1,100d filename
 ##### Remove lines with string (e.g. 'bbo')
 ```bash
 sed "/bbo/d" filename
-- case insensitive:
+# case insensitive:
 sed "/bbo/Id" filename
 ```
+
 ##### Remove lines whose nth character not equal to a value (e.g. 5th character not equal to 2)
 ```bash
 sed -E '/^.{5}[^2]/d'
@@ -505,12 +551,12 @@ sed -i '$ s/.$//' filename
 
 ##### Add string to beginning of file (e.g. "\[")
 ```bash
-sed -i '1s/^/[/' file
+sed -i '1s/^/[/' filename
 ```
 
 ##### Add string at certain line number (e.g. add 'something' to line 1 and line 3)
 ```bash
-sed -e '1isomething -e '3isomething'
+sed -e '1isomething' -e '3isomething'
 ```
 
 ##### Add string to end of file (e.g. "]")
@@ -524,7 +570,7 @@ sed '$a\'
 
 ##### Add string to beginning of every line (e.g. 'bbo')
 ```bash
-sed -e 's/^/bbo/' file
+sed -e 's/^/bbo/' filename
 ```
 
 ##### Add string to end of each line (e.g. "}")
@@ -537,7 +583,14 @@ sed -e 's/$/\}\]/' filename
 sed 's/.\{4\}/&\n/g'
 ```
 
-##### Concatenate/combine/join files with a seperator and next line (e.g separate by ",")
+##### Add a line after the line that matches the pattern (e.g. add a new line with "world" after the line with "hello")
+```bash
+sed '/hello*/a world' filename
+# hello
+# world
+```
+
+##### Concatenate/combine/join files with a separator and next line (e.g separate by ",")
 ```bash
 sed -s '$a,' *.json > all.json
 ```
@@ -645,7 +698,7 @@ sed '$ s/.$//'
 
 ##### Insert character at specified position of file (e.g. AAAAAA --> AAA#AAA)
 ```bash
-sed -r -e 's/^.{3}/&#/' file
+sed -r -e 's/^.{3}/&#/' filename
 ```
 
 
@@ -700,7 +753,7 @@ awk -v N=7 '{print}/bbo/&& --N<=0 {exit}'
 
 ##### Print filename and last line of all files in directory
 ```bash
-ls|xargs -n1 -I file awk '{s=$0};END{print FILENAME,s}' file
+ls|xargs -n1 -I file awk '{s=$0};END{print FILENAME,s}' filename
 ```
 
 ##### Add string to the beginning of a column (e.g add "chr" to column $3)
@@ -710,12 +763,12 @@ awk 'BEGIN{OFS="\t"}$3="chr"$3'
 
 ##### Remove lines with string (e.g. 'bbo')
 ```bash
-awk '!/bbo/' file
+awk '!/bbo/' filename
 ```
 
 ##### Remove last column
 ```bash
-awk 'NF{NF-=1};1' file
+awk 'NF{NF-=1};1' filename
 ```
 
 ##### Usage and meaning of NR and FNR
@@ -774,13 +827,13 @@ awk '{printf("%s\t%s\n",NR,$0)}'
 
 ##### Break combine column data into rows
 ```bash
-# For example, seperate the following content:
+# For example, separate the following content:
 # David    cat,dog
 # into
 # David    cat
 # David    dog
 
-awk '{split($2,a,",");for(i in a)print $1"\t"a[i]}' file
+awk '{split($2,a,",");for(i in a)print $1"\t"a[i]}' filename
 
 # Detail here:　http://stackoverflow.com/questions/33408762/bash-turning-single-comma-separated-column-into-multi-line-string
 ```
@@ -982,6 +1035,11 @@ find . -type f -empty
 find . -type f -empty -delete
 ```
 
+##### Recursively count all the files in a directory
+```bash
+find . -type f | wc -l
+```
+
 ## Condition and loop
 [[back to top](#handy-bash-one-liners)]
 
@@ -991,7 +1049,7 @@ find . -type f -empty -delete
 if [[ "$c" == "read" ]]; then outputdir="seq"; else outputdir="write" ; fi
 
 # Test if myfile contains the string 'test':
-if grep -q hello myfile; then …
+if grep -q hello myfile; then echo -e "file contains the string!" ; fi
 
 # Test if mydir is a directory, change to it and do other stuff:
 if cd mydir; then
@@ -1000,9 +1058,13 @@ else
   echo >&2 "Fatal error. This script requires mydir."
 fi
 
-# if variable is null
-if [ ! -s "myvariable" ]
-#True of the length if "STRING" is zero.
+# Check if a variable is null
+if [ -z "$var" ]; then echo "NULL"; else echo "Not NULL"; fi
+# or 
+[ -z "$var" ] && echo "NULL"
+
+# Using test command (same as []), to test if the length of variable is nonzero
+test -n "$myvariable" && echo myvariable is "$myvariable" || echo myvariable is not set
 
 # Test if file exist
 if [ -e 'filename' ]
@@ -1017,16 +1079,17 @@ then
 fi
 
 # Test if the value of x is greater or equal than 5
-if [ "$x" -ge 5 ]; then …
+if [ "$x" -ge 5 ]; then echo -e "greater or equal than 5!" ; fi
 
 # Test if the value of x is greater or equal than 5, in bash/ksh/zsh:
-if ((x >= 5)); then …
+if ((x >= 5)); then echo -e "greater or equal than 5!" ; fi
 
 # Use (( )) for arithmetic operation
-if ((j==u+2))
+if ((j==u+2)); then echo -e "j==u+2!!" ; fi
 
 # Use [[ ]] for comparison
-if [[ $age -gt 21 ]]
+if [[ $age -gt 21 ]]; then echo -e "forever 21!!" ; fi
+
 ```
 
 [More if commands](http://tldp.org/LDP/Bash-Beginners-Guide/html/sect_07_01.html)
@@ -1106,6 +1169,45 @@ date +%F
 # or
 date +'%d-%b-%Y-%H:%M:%S'
 # 10-Apr-2020-21:54:40
+
+# Returns the current time with nanoseconds.
+date +"%T.%N"
+# 11:42:18.664217000  
+
+# Get the seconds since epoch (Jan 1 1970) for a given date (e.g Mar 16 2021)
+date -d "Mar 16 2021" +%s
+# 1615852800
+# or
+date -d "Tue Mar 16 00:00:00 UTC 2021"  +%s
+# 1615852800  
+
+# Convert the number of seconds since epoch back to date
+date --date @1615852800
+# Tue Mar 16 00:00:00 UTC 2021
+
+```
+
+##### Print current time point for N days ago or N days after
+```bash
+# print current date first (for the following example)
+date +"%F %H:%M:%S"
+# 2023-03-11 16:17:09
+
+# print the time that is 1 day ago
+date -d"1 day ago" +"%F %H:%M:%S"
+# 2023-03-10 16:17:09
+
+# print the time that is 7 days ago
+date -d"7 days ago" +"%F %H:%M:%S"
+# 2023-03-04 16:17:09
+
+# print the time that is a week ago
+date -d"1 week ago" +"%F %H:%M:%S"
+# 2023-03-04 16:17:09
+
+# add 1 day to date
+date -d"-1 day ago" +"%F %H:%M:%S"
+# 2023-03-12 16:17:09
 ```
 
 ##### wait for random duration (e.g. sleep 1-5 second, like adding a jitter)
@@ -1456,9 +1558,17 @@ stat filename.txt
 ps aux
 ```
 
+##### List processes by top memory usage
+```bash
+ps -eo pid,ppid,cmd,%mem,%cpu --sort=-%mem | head
+```
+
 ##### Display a tree of processes
 ```bash
 pstree
+
+# or
+ps aux --forest
 ```
 
 ##### Find maximum number of processes
@@ -1707,6 +1817,18 @@ du -h
 du -sk /var/log/* |sort -rn |head -10
 ```
 
+##### check the Inode utilization
+```
+df -i
+# Filesystem      Inodes  IUsed   IFree IUse% Mounted on
+# devtmpfs        492652    304  492348    1% /dev
+# tmpfs           497233      2  497231    1% /dev/shm
+# tmpfs           497233    439  496794    1% /run
+# tmpfs           497233     16  497217    1% /sys/fs/cgroup
+# /dev/nvme0n1p1 5037976 370882 4667094    8% /
+# tmpfs           497233      1  497232    1% /run/user/1000
+```
+
 ##### Show all file system type
 ```bash
 df -TH
@@ -1852,7 +1974,7 @@ ldconfig -p
 ldd /bin/ls
 ```
 
-##### Check user login
+##### Check the most recent login of all users
 ```bash
 lastlog
 ```
@@ -2002,11 +2124,6 @@ killall pulseaudio
 # then press Alt-F2 and type in pulseaudio
 ```
 
-##### When sound not working
-```bash
-killall pulseaudio
-```
-
 ##### List information about SCSI devices
 ```bash
 lsscsi
@@ -2163,13 +2280,19 @@ sar -n ALL
 
 # reading SAR log file using -f
 sar -f /var/log/sa/sa31|tail
+```
 
 ##### Reading from journal file
 ```bash
 journalctl --file ./log/journal/a90c18f62af546ccba02fa3734f00a04/system.journal  --since "2020-02-11 00:00:00"
 ```
 
-##### Show a listing of last logged in users.
+##### Show a listing of last logged in users
+```bash
+last
+```
+
+##### Show a listing of unsuccessful (bad) login attempts 
 ```bash
 lastb
 ```
@@ -2347,6 +2470,11 @@ dig +short www.example.com
 host www.example.com
 ```
 
+##### Check public IP address
+```bash
+curl http://checkip.amazonaws.com
+```
+
 ##### Get DNS TXT record a of domain
 ```bash
 dig -t txt www.example.com
@@ -2380,14 +2508,14 @@ nc -vw5 google.com 22
 # From server A:
 $ sudo nc -l 80
 # then you can connect to the 80 port from another server (e.g. server B):
-# e.g. telent <server A IP address> 80
+# e.g. telnet <server A IP address> 80
 # then type something in server B
 # and you will see the result in server A!
 ```
 
 ##### Check which ports are listening for TCP connections from the network
 ```bash
-#notice that some companies might not like you using nmap
+# note that some companies might not like you using nmap
 nmap -sT -O localhost
 
 # check port 0-65535
@@ -2395,7 +2523,7 @@ nmap  -p0-65535 localhost
 ```
 ##### Check if a host is up and scan for open ports, also skip host discovery.
 ```bash
-#skips checking if the host is alive which may sometimes cause a false positive and stop the scan.
+# skips checking if the host is alive. this may sometimes cause a false positive, stopping the scan.
 $ nmap google.com -Pn
 
 # Example output:
@@ -2417,7 +2545,7 @@ $ nmap -A -T4 scanme.nmap.org
 # -A to enable OS and version detection, script scanning, and traceroute; -T4 for faster execution
 ```
 
-##### Look up website information (e.g. name server), searches for an object in a RFC 3912 database.
+##### Look up website information (e.g. name server), searches for an object in a RFC 3912 database
 ```bash
 whois google.com
 ```
@@ -2427,7 +2555,7 @@ whois google.com
 openssl s_client -showcerts -connect www.example.com:443
 ```
 
-##### Display IP address
+##### Display network interfaces and their associated IP addresses
 ```bash
 ip a
 ```
@@ -2487,6 +2615,10 @@ curl -I http://example.com/
 # Accept-Ranges: bytes
 # Vary: Accept-Encoding
 ```
+##### Find out the time spent between request and response
+```
+curl -v -o /dev/null -s -w 'Total: %{time_total}s\n' google.com
+```
 
 ##### Find out the http status code of a URL
 ```bash
@@ -2522,16 +2654,16 @@ sudo iptables –A INPUT –s <IP> -p tcp –dport 80 –j DROP
 ```bash
 # If file is not specified, the file /usr/share/dict/words is used.
 look phy|head -n 10
-# Phil
-# Philadelphia
-# Philadelphia's
-# Philby
-# Philby's
-# Philip
-# Philippe
-# Philippe's
-# Philippians
-# Philippine
+# phycic
+# Phyciodes
+# phycite
+# Phycitidae
+# phycitol
+# phyco-
+# phycochrom
+# phycochromaceae
+# phycochromaceous
+# phycochrome
 ```
 
 ##### Repeat printing string n times (e.g. print 'hello world' five times)
@@ -2589,6 +2721,40 @@ sdiff fileA fileB
 diff fileA fileB --strip-trailing-cr
 ```
 
+##### Find common/differing lines
+```bash
+# having two sorted and uniqed files (for example after running `$ sort -uo fileA fileA` and same for fileB):
+# ------
+# fileA:
+# ------
+# joey
+# kitten
+# piglet
+# puppy
+# ------
+# fileB:
+# ------
+# calf
+# chick
+# joey
+# puppy
+#
+# Find lines in both files
+comm -12 fileA fileB
+# joey
+# puppy
+#
+# Find lines in fileB that are NOT in fileA
+comm -13 fileA fileB
+# calf
+# chick
+#
+# Find lines in fileA that are NOT in fileB
+comm -23 fileA fileB
+# kitten
+# piglet
+```
+
 ##### Number a file (e.g. fileA)
 
 ```bash
@@ -2733,7 +2899,7 @@ echo -e 'text here \c'
 
 ##### View first 50 characters of file
 ```bash
-head -c 50 file
+head -c 50 filename
 ```
 
 ##### Cut and get last column of a file
@@ -2753,12 +2919,33 @@ var=$((var+1))
 cat filename|rev|cut -f1|rev
 ```
 
-##### Cat to a file
+##### Create or replace a file with contents
 ```bash
 cat >myfile
 let me add sth here
-exit by control + c
-^C
+# exit with ctrl+d
+
+# or using tee
+tee myfile
+let me add sth else here
+# exit with ctrl+d
+```
+
+##### Append to a file with contents
+```bash
+cat >>myfile
+let me add sth here
+# exit with ctrl+d
+
+# or
+cat << EoF >> filename
+> add something here
+> EoF 
+
+# or using tee
+tee -a myfile
+let me add sth else here
+# exit with ctrl+d
 ```
 
 ##### Clear the contents of a file (e.g. filename)
@@ -2773,7 +2960,7 @@ echo 'hihi' >>filename
 
 ##### Working with json data
 ```bash
-#install the useful jq package
+# Install the useful jq package
 # sudo apt-get install jq
 # e.g. to get all the values of the 'url' key, simply pipe the json to the following jq command(you can use .[]. to select inner json, i.e jq '.[].url')
 cat file.json | jq '.url'
@@ -2848,7 +3035,7 @@ while read a b; do yes $b |head -n $a ;done <test.txt
 ## Others
 [[back to top](#handy-bash-one-liners)]
 
-##### Describe the format and characteristics of image files.
+##### Describe the format and characteristics of image files
 ```bash
 identify myimage.png
 #myimage.png PNG 1049x747 1049x747+0+0 8-bit sRGB 1.006MB 0.000u 0:00.000
@@ -2877,6 +3064,13 @@ cal
 # only display November
 cal -m 11
 ```
+
+##### Convert the hexadecimal MD5 checksum value into its base64-encoded format.
+```bash
+openssl md5 -binary /path/to/file| base64
+# NWbeOpeQbtuY0ATWuUeumw==
+```
+
 ##### Forces applications to use the default language for output
 ```bash
 export LC_ALL=C
@@ -3014,11 +3208,40 @@ rsync -av directory user@ip_address:/path/to/directory.bak
 # skip files that are newer on receiver (i prefer this one!)
 ```
 
+##### Create a temporary directory and `cd` into it
+```bash
+cd $(mktemp -d)
+# for example, this will create a temporary directory "/tmp/tmp.TivmPLUXFT"
+```
+
 ##### Make all directories at one time!
 ```bash
 mkdir -p project/{lib/ext,bin,src,doc/{html,info,pdf},demo/stat}
 # -p: make parent directory
-# this will create project/doc/html/; project/doc/info; project/lib/ext ,etc
+# this will create:
+# project/
+# project/bin/
+# project/demo/
+# project/demo/stat/
+# project/doc/
+# project/doc/html/
+# project/doc/info/
+# project/doc/pdf/
+# project/lib/
+# project/lib/ext/
+# project/src/
+#
+# project/
+# ├── bin
+# ├── demo
+# │   └── stat
+# ├── doc
+# │   ├── html
+# │   ├── info
+# │   └── pdf
+# ├── lib
+# │   └── ext
+# └── src
 ```
 
 ##### Run command only if another command returns zero exit status (well done)
@@ -3088,6 +3311,12 @@ scp -r directoryname user@ip:/path/to/send
 # :(){:|:&};:
 ```
 
+##### Trigger kernel crash
+```bash
+# Don't try this at home! 
+echo c > /proc/sysrq-trigger
+```
+
 ##### Use the last argument
 ```bash
 !$
@@ -3098,7 +3327,7 @@ scp -r directoryname user@ip:/path/to/send
 echo $?
 ```
 
-##### Extract .xf
+##### Extract .xz
 ```
 unxz filename.tar.xz
 # then
@@ -3138,8 +3367,7 @@ yes n
 # or 'anything':
 yes anything
 
-# For example:
-```bash
+# pipe yes to other command
 yes | rm -r large_directory
 ```
 
@@ -3150,9 +3378,9 @@ fallocate -l 10G 10Gigfile
 
 ##### Create dummy file of certain size (e.g. 200mb)
 ```bash
-dd if=/dev/zero of=//dev/shm/200m bs=1024k count=200
+dd if=/dev/zero of=/dev/shm/200m bs=1024k count=200
 # or
-dd if=/dev/zero of=//dev/shm/200m bs=1M count=200
+dd if=/dev/zero of=/dev/shm/200m bs=1M count=200
 
 # Standard output:
 # 200+0 records in
@@ -3165,9 +3393,29 @@ dd if=/dev/zero of=//dev/shm/200m bs=1M count=200
 watch -n 1 wc -l filename
 ```
 
+##### Use Bash Strict Mode
+```bash
+# These options can make your code safer but, depending on how your pipeline is written, it might be too aggressive 
+# or it might not catch the errors that you are interested in
+
+# for reference see https://gist.github.com/mohanpedala/1e2ff5661761d3abd0385e8223e16425
+#               and https://mywiki.wooledge.org/BashPitfalls#set_-euo_pipefail
+
+set -o errexit      # exit immediately if a pipeline returns a non-zero status
+set -o errtrace     # trap ERR from shell functions, command substitutions, and commands from subshell
+set -o nounset      # treat unset variables as an error
+set -o pipefail     # pipe will exit with last non-zero status, if applicable
+set -Eue -o pipefail  # shorthand for above (pipefail has no short option)
+```
+
 ##### Print commands and their arguments when execute (e.g. echo `expr 10 + 20 `)
 ```bash
 set -x; echo `expr 10 + 20 `
+# or
+set -o xtrace; echo `expr 10 + 20 `
+
+# to turn it off..
+set +x
 ```
 
 ##### Print some meaningful sentences to you (install fortune first)
@@ -3198,6 +3446,9 @@ q -d "," "select c3,c4,c5 from /path/to/file.txt where c3='foo' and c5='boo'"
 # Create session and attach:
 screen
 
+# Create a screen and name it 'test'
+screen -S test
+
 # Create detached session foo:
 screen -S foo -d -m
 
@@ -3216,15 +3467,16 @@ screen -r foo
 # Kill session foo:
 screen -r foo -X quit
 
+
 # Scroll:
-Hit your screen prefix combination (C-a / control+A), then hit Escape.
-Move up/down with the arrow keys (↑ and ↓).
+# Hit your screen prefix combination (C-a / control+A), then hit Escape.
+# Move up/down with the arrow keys (↑ and ↓).  
 
 # Redirect output of an already running process in Screen:
- (C-a / control+A), then hit 'H'
+# (C-a / control+A), then hit 'H'  
 
 # Store screen output for Screen:
-Ctrl+A, Shift+H
+# Ctrl+A, Shift+H  
 # You will then find a screen.log file under current directory.  
 ```
 
@@ -3351,6 +3603,8 @@ source .venv/bin/activate
 type pip
 # 5. Now you can install your pip package, here requirements.txt is simply a txt file containing all the packages you want. (e.g tornado==4.5.3).
 pip install -r requirements.txt
+# 6. Exit virtual environment
+deactivate
 ```